Fuel/Air Ratio Control
Using a zero governor (atmospheric regulator)
A zero governor (atmospheric regulator) is a ratio regulator that relies on suction from an aspirator mixer to control air/fuel ratio. It is the classic way of controlling fuel air ratio in premix combus- tion system as it does not require high gas pressure supply. This is very important when gas delivery systems have limited supply pressures. Zero governors work by maintaining atmospheric pressure (zero pressure) at the regulator outlet. Increasing air flow through a mixers' venturi increases suction on gas line. To maintain its "zero" outlet pressure, the regulator opens its' gas valve to increase the amount of gas flowing to the mixer. When the air flow is reduced, the suction is decreased and the regulator valve closes to maintain the zero outlet pressure.
Zero Governor (Ratio Regulator)
3065 Mixer
Burner
Vent Connection
Air Valve
Figure 3. Typical ratio control system with zero governor ratio regulator
Using a cross-connected ratio regulator
Gas Line
There are a number of reasons to cross-connect the ratio regulator feeding a 3065:
Zero Governor (Ratio Regulator)
3065 Mixer
Burner
— The combustion chamber pressure varies or is different than the room pressure. For example premix burner used as pilot tips are almost always cross connected. — The air pressure drop across the mixer is less than 66% of the air supply pressure, there may not be enough suction generated by the mixer to use a true zero governor. The regulator can be cross-connected to the mixture pressure tap or air pressure tap with an impulse line (ideally 3/8" copper or SST tubing). When a zero governor is cross-connected, the gas pressure at the zero governor (regulator) inlet must be at least equal to the impulse line pressure plus pressure drop across the governor. Do not set the gas pressure upstream of the ratio regulator higher than necessary. When the gas pressure is increased above optimum the air/fuel ratio adjustment will lose resolution and get touchy. Cross-connecting to the mixture pressure generally provides better ratio control resolution than using the air pressure tap. Use the air pressure tap to impulse line and regulator case is objectionable. For example when the fuel contains corrosive components.
Air Valve
Impulse Line on Mixture Pres. Tap
Figure 3b. Typical ratio control system with zero governor ratio regulator cross-connected to the mixture pressure.
Gas Line
Zero Governor (Ratio Regulator)
3065 Mixer
Burner
Impulse Line on Air Pres. Tap
Air Valve
Figure 3c. Typical ratio control system with zero governor ratio regulator cross-connected to the air pressure.
Bulletin 3065 page 4
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